[Python-Dev] misbehaving contains (original) (raw)

Raymond Hettinger python at rcn.com
Wed Jan 23 01:40:16 CET 2008

[Daniel Stutzbach]

There are many places in the C implementation where a slot returns an int rather than a PyObject. There other replies in this thread seem to support altering the signature of the slot to return a PyObject. Is this setting a precedent that all slots should return a PyObject?

I hope not.

One cost of these hypergeneralizations is that it makes Python slower.

Also, it makes it harder to write code when you can no longer count on simple predicates to return either True or False.

[tomer filiba]

i see no reason it should behave differently than eq and the rest of comparison operators

The rest of the equality operators were a special case to allow rich comparisons which were needed to implement some vector arithmetic idioms.

It was thought that those use case had enough benefit to pay for the negative impacts:

Does making contains return a PyObject have use cases that are worth those costs?

It is reminiscent of the ill-fated proposals to overrride 'and', 'or', 'is', and 'is not'. A couple more questions about the proposal:

If contains returns 17 in Tomer's example, what would "8 not in f" return?

And, would we lose the nice relationship expressed by:

for elem in container:
    assert elem in container

It looks like changing the semantics would introduce some weirdness.

Raymond

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